The breath of life

You only need to stick your head beneath the bathwater for a minute or so to realise how important breathing is for us. We use the oxygen, which makes up about one-fifth of the air we breathe, to fuel our bodies by burning (oxidising) our food. We cannot do without oxygen for long, but there is a surprising variety of organisms that can and which inhabit environments where oxygen is in short supply or absent altogether. Habitats in which oxygen is at low concentrations (microaero-bic) or absent (anaerobic or anoxic) are in fact quite common. They include: the muds and sediments of lakes, rivers, ponds and oceans; bogs, swamps, deep waters and waterlogged soils; compost heaps and sewage treatment plants; and the intestines of animals and some deep underground areas. These habitats are anoxic because they are remote from contact with the air or the oxygen they contained has been used up by the activities of organisms.

Anaerobic conditions prevailed during the early stages of the Earth's history. The oxygen in the atmosphere originated from the activities of photosynthetic organisms and only accumulated once the quantity of oxygen produced by photosynthesis exceeded the capacity of chemicals in soils and sediments to remove it (by being oxidised). Organisms which live in aerobic conditions (in the presence of oxygen) had to adapt to doing so, while strictly anaerobic organisms, which cannot survive exposure to oxygen, have persisted in those habitats where it is absent. Strictly anaerobic microorganisms cannot survive exposure to oxygen because it forms some highly toxic reactive ions and molecules. Among these are the superoxide ion (O-), hydrogen peroxide (H2O2) and the hydroxyl radical (OH-). Aerobic organisms have enzymes which destroy these toxic products of oxygen. These include catalase and peroxidase, which break down hydrogen peroxide, and superoxide dismutase, which destroys the superoxide ion. Microorganisms which are strictly anaerobic are killed by contact with oxygen since they lack these mechanisms for dealing with its toxic products.

There are many groups of strictly anaerobic bacteria, a few fungi and a few protozoa. Some of these use fermentation reactions to gain energy from their food. These reactions are, of course, anaerobic, but involve the first steps of the pathways used by aerobic organisms. Anaerobic organisms can only release part of the energy of their food. They only gain 1/18 of the energy released by an aerobic organism which can break down a sugar to water and carbon dioxide by oxidising it through the involvement of oxygen. It is only the fact that they are exploiting habitats where food, in the form of organic material, is plentiful (such as in a compost heap) that they are able to survive in spite of such inefficient food utilisation. They also have less competition for the food, since aerobic organisms are excluded. Since anaerobes do not break down their food completely, they excrete much more complex molecules than do aerobic organisms (which produce carbon dioxide and water as the end products of sugar metabolism). Different groups of microorganisms produce different end products including: ethanol, acetone, glycerol, lactic acid, butyric acid, hydrogen and methane. Some anaerobic bacteria use inorganic materials, such as sulphates, as their energy sources.

Growing plants are unlikely to be faced with anaerobic conditions since they produce oxygen as one of the products of photosynthesis. However, dormant stages (pollen, seeds and spores) can survive without oxygen for some time. Many animals can survive periods of exposure to anoxic conditions, at least in some parts of their bodies. We can survive such conditions ourselves. If we exercise vigorously, the blood cannot supply enough oxygen to the muscles for aerobic respiration and they begin to respire anaerobically. Lactic acid is produced as an end product which, if it accumulates, causes muscle fatigue and pain. Many invertebrates can survive periods of anoxia, switching to anaerobic metabolism and accumulating organic end products which, if oxygen returns, can be further metabolised.

There are a variety of groups of parasitic worms which live in the

212 LIFE AT THE LIMITS

intestine of mammals and other vertebrates. These include nematodes (roundworms), cestodes (tapeworms), trematodes (flukes) and acan-thocephalans (spiny-headed worms). The wall of their host's intestine is supplied with blood which carries oxygen, but, in the centre of the lumen, conditions are likely to be anoxic. The amount of oxygen available to a parasite will thus depend on its position within the intestine. These parasites have no respiratory or circulatory systems and rely on diffusion for the transport of respiratory gases and other substances. Some of the parasites are quite large. Ascaris lumbricoides, for example, which is a common nematode parasite of humans in developing countries, is up to 30 centimetres long. Even if there is some oxygen available to them, it is likely that the deeper tissues of the parasite will be anoxic because of their reliance on diffusion for the transport of oxygen. Although they possess the pathways which would enable them to metabolise sugars aerobically, they, in fact, utilise anaerobic pathways. Like anaerobic bacteria, they can afford to use these inefficient anaerobic pathways since they have plenty of food available to them in the intestine of their host.

Long-term survival of anoxia by animals is rare, but there are a few invertebrates which can achieve this feat. James Clegg from the University of California's Bodega Marine Laboratory has hatched the brine shrimp Artemia from cysts stored in anoxic conditions for four years (Figure 6.4). Their food reserves (trehalose, glycogen and glycerol) showed no decline during this period, indicating that they had ceased metabolising. Clegg estimates that he should have been able to detect a level of metabolism just 0.002 per cent of normal. The cysts thus survive in a state of cryptobiosis as a result of anoxia (anoxybiosis). This is a similar phenomenon to anhydrobiosis, which is a state of cryptobiosis induced by desiccation (see Chapter 3). It is all the more remarkable since metabolism ceases, even though the cysts are fully hydrated and are at room temperature. Artemia cysts will survive a number of severe environmental stresses including desiccation, osmotic stress, ultraviolet (UV) radiation and temperature extremes, as well as anoxia. A protective cyst wall, the accumulation of trehalose

figure 6.4 Scanning electron micrograph of cysts of the brine shrimp Artemia. The cysts are about 0.2 centimetres in diameter.

and glycerol, and the production of a heat-stress protein which acts as a molecular chaperone and of artemin (which is another protein specific to the cyst stage) may all be involved in its remarkable survival abilities. The survival of prolonged anoxia has also been reported in sponge gemmules (for four months), which are the protected dispersal stages of sponges, and in some nematodes (for 30 days). It may be much more widespread than we realise.

too much sun

I guess we are all aware that we should not spend too long in the sun, and that we must protect ourselves with clothing and sunscreens, if we do not want to end up looking like a boiled lobster. We are particularly aware of this in New Zealand, given our relatively low latitude (45° South) and proximity to the Ozone Hole which develops annually over Antarctica. Ozone levels over New Zealand have decreased by more than 5 per cent over the past 16 years. The sun gives the Earth life, via photosynthesis, but too much solar radiation is damaging to living

organisms and they need to cope with its harmful effects. The main destructive component is UV radiation. This is the part of the electromagnetic spectrum that has shorter wavelengths, and higher frequencies, than those of visible light (Figure 6.5). X-rays and gamma rays have shorter wavelengths still. The sun produces three classes of UV radiation: UVA (wavelengths of 400-315 nanometres), UVB (315-280 nanometres) and UVC (280-100 nanometres). UVC does not reach the surface of the Earth and UVB is the radiation that is responsible for most of the harmful effects on organisms, since it is the form that they most easily absorb. Not all radiation is bad, of course, and almost all life on Earth depends ultimately on the energy supplied by solar radiation in the visible range of the electromagnetic spectrum (400-700 nanometres).

The harmful effects of UV radiation (and other radiation, like X-rays) results from its high energy levels. When the radiation penetrates a living organism, it interacts with its molecules, resulting in damage and in the formation of ions and free radicals. These are highly reactive forms of atoms and molecules that will react with the molecules of cells to produce damaging effects. Of particular importance is the effect of radiation on chromosomes. The absorption of UV radiation by DNA damages genes, either directly or through their interactions with ions and free radicals, causing mutations. The damage may be repairable, but, if it is too great, there are disastrous consequences for the organ ism. Most mutations are harmful and may result in the cell dying or becoming cancerous. This process occurs, for example, in the development of skin melanomas as a result of too much exposure to the sun.

Earth's organisms are protected from most of the UV radiation from the sun since the atmosphere absorbs it. Ozone (O3, one of the molecular forms of oxygen) is formed by the interaction between UV radiation and normal molecular oxygen (O2). The absorption of solar radiation by O3 and O2 prevents almost all radiation with a wavelength of less than 290 nanometres from reaching the surface of the Earth - which is just as well, since if it did reach the Earth it would kill most of its organisms. This is why there is so much concern about the destruction of ozone in the atmosphere by chemicals which react with it (such as chlorofluoro-carbons).

Despite the shielding effects of the atmosphere, significant amounts of UVA and UVB reach the surface of the Earth. Organisms may be sheltered from the sun under rocks, water or the surface of the soil. Hairs, feathers, scales, spines, skin and cuticle also provide some protection to the cells beneath. The skin itself is protected by melanin, a dark pigment which absorbs the radiation. The cuticles of insects, and other terrestrial invertebrates, contain dark pigments too and, in plants and some microorganisms, pigments may provide some protection. Many organisms have mechanisms that repair the damage done by the absorption of UV radiation. Radiation-induced breaks in DNA molecules are removed and DNA resynthesised by using the undamaged strand as a template (excision or dark repair). Some repair mechanisms depend on the presence of light, a process referred to as photoreactivation (light repair). The damage caused by UV radiation is thus lessened considerably if the organism is subsequently exposed to light in the visible range of the spectrum. In plants that are exposed to high levels of solar radiation - for example plants at high altitude -these repair mechanisms must be operating almost continuously.

Apart from solar radiation, organisms are exposed to other sources of radiation from terrestrial and extraterrestrial sources. Natural sources of terrestrial radiation derive from the decay of radioactive materials in the rocks, soil, air and water. In the past 50 years or so, there have been increasing sources of radiation from human-generated sources such as nuclear power plants and from radionucleotides used for medical diagnosis and therapy. Exposure to these radiations is normally low, except in some artificial situations. Non-solar extraterrestrial radiation consists mainly of cosmic rays which are high-energy particles originating outside our solar system. Any cells struck by one of these particles will be destroyed, but such collisions are rare. Damaged cells are easily replaced by plants and animals, and by the reproduction of surviving microorganisms.

Some microorganisms are able to tolerate high levels of radiation exposure. Micrococcus radiodurans is a radiation-resistant bacterium which was first isolated from a can of meat that had been exposed to high levels of gamma radiation to sterilise it. Another bacterium, Pseudomonas radiodurans, survives in the cooling waters of nuclear reactors. These organisms can survive high levels of radiation exposure because they have very efficient mechanisms for DNA repair.